This invention relates to a more energy efficient aeration of pond waters for the purpose of increasing the exchange of gases between the atmosphere and water. Using this invention the oxygen uptake into the total water volume of a pond can be increased significantly, and this increase in oxygen availability accelerates biological activity within the pond. Improved capacity for biological activity subsequently improves water quality, minimizes odor, and improves ultimate water quality. The invention is primarily but not exclusively targeted at increasing the biological degradation of organics in wastewater.
The treatment of wastewaters in surface impoundments can be performed by a method commonly referred to as an “extended aeration pond”, also called “facultative ponds”, wherein, the wastewater is allowed to settle in a pond, which is typically deeper than six meters. The denser wastewater (20) and its contained suspended solids normally sink to the bottom of the pond, and because of their high concentration and subsequent absence of free oxygen, then commence to degrade anaerobically. This anaerobic bottom layer (20) is relatively thin and significantly more dense than the upper layer (21) that contains the bulk of the pond's water, which is ideally and predominately engaged in “aerobic processes”. It is in this upper aerobic layer that microbes consume dissolved oxygen to convert much of the organics in the wastewater into gases such as nitrogen and carbon dioxide, which ultimately escape from the water into the atmosphere. These decomposable organics are referred to and reported in water quality testing as “biological oxygen demand” (BOD).
For aerobic processes to function efficiently on an industrial scale, large volumes of gasses need to be both absorbed into the water and released from it. This gaseous exchange rate is a limiting factor in maximizing removal of BOD in wastewater, and this invention is targeted at accelerating the gas exchange that is achievable at the surface of a pond. The capacity of this invention to decrease BOD levels is logically limited by the pond's surface area and not a linear function of the minimal volume of compressed air used by the invention's airlift pump, however, the BOD reduction capacity using this invention is several times that of natural ponds. To fully maximize the air/water surface area and hence the gas exchange rates, many wastewater facilities inject very large volumes of compressed air into bubbling systems capable of achieving highly turbulent conditions. When comparing energy usage per mass of BOD destroyed, these techniques are very high-energy alternatives.
The main mechanism for gas interchange between water and air occurs at the surface (23) of the two phases. The exchange rate is roughly proportional to the surface area and to the difference in the partial gas pressures between the two phases (23, 24). For example, the partial pressure of oxygen in air is approximately 0.18 atmospheres, which is about the same partial pressure of water containing 8 ppm of oxygen. Therefore, surface waters, which contain typically 7 ppm of oxygen are almost saturated and cannot be expected to rapidly absorb additional oxygen. Similarly, water with an oxygen partial pressure of one ppm would be expected to absorb oxygen at seven times the rate of water containing 7 ppm of oxygen.
By bubbling the air at greater distance below the surface, the partial pressures can be increased but this requires greater compressed air pressures and increased energy consumption. e.g. At a depth of 11 meters the pressure is two atmospheres, and the water should equilibrate to about 16 ppm. Unless the bubbles are unusually small, the bubbles rise rapidly to the surface and their opportunity for gaseous exchange is brief, e.g. at an bubble injection depth of 6 meters only 12% of the available oxygen is absorbed before the bubbles reach the surface. Thus extended aeration ponds that depend on bubbling air need to pump about eight times the actual volume of air required.
Another means of increasing the oxygen uptake per hectare of pond area is to artificially increase the water's surface area by agitation or by spraying water into the air. While effective, this is limited in its efficiency because surface waters used in the process are already close to maximum oxygen saturation, and the droplets have limited flight time. Typically, these processes result in the aerobic biological activity restricted predominately to the surface waters, in the upper 0.5 meters, and the oxygen concentrations typically decrease to very low levels after only one meter of depth.
Alternatively, water from the lower depths of the pond can be pumped to the surface, but care needs to be taken so as not to disturb the necessary stratification containing the anaerobic biological process. Pumping the water to the surface from the bottom of the pond's aerobic layer works well because the deeper water is almost devoid of free oxygen, and even without spraying it into the air, the action of simply spreading it on the surface allows wind and gentle wave action to provide an effective exchange mechanism. Thus, the principle here is to improve gas exchange rates by exposing water to air, such that the difference in partial pressure is maximized; there are many effective sewage treatment facilities that depend on this principle.
This invention combines the above concepts in conjunction with a novel means of using an airlift pump to transport large volumes of deep pond water to the surface. Thus, the pond gains some oxygen from the bubble aeration of the airlift pump, but much more from exposing large volumes of low oxygen content water to the pond's surface to facilitate natural adsorption of oxygen. This also effectively provides high mass turnover rates of the top five or more meters of surface waters, thus extending the working aerobic volume by an order of magnitude, which has a positive impact on the overall BOD oxidation capacity.
While the use of airlift pumps in wastewater treatment facilities is almost ubiquitous, their application is predominately for pumping of water and slurries to an elevated level. Some plants use bubbler aeration and some have enclosed the bubbler in a duct to induce a pumping effect, in order to move the liquid around enclosed circuits. Furthermore, airlift pump designs currently used in wastewater treatment ponds for circulation, almost completely oxygenate the water as it rises in the airlift pump, thus the water pumped to the surface is now close to saturation and will only absorb minimal amounts of the additional oxygen available at the surface. This invention is designed to an entirely different set of criteria, that of maximizing the volume of water from a given amount of pressurized air (25), where the oxygen transfer from the bubbling supplies less. than 50% of the total oxidative activity imparted to the pond, and the remaining oxygenation is a result of exposing large volumes of water, with a low partial-pressure of oxygen to the pond's surface.